Until about 30 years ago, atmospheric scientists believed that all of the ozone in the lower atmosphere (troposphere) intruded from the
upper atmosphere (stratosphere), where it formed by the action of sunlight on oxygen molecules. The work of atmospheric chemists during the
1970s dramatically altered that view. Now we understand that more than half of the ozone in the troposphere comes from chemical interactions
within the troposphere itself.

In the stratosphere, ozone shields us from the Sun’s deadly ultraviolet radiation. But in the troposphere, this same gas impairs lung
capacity and reduces agricultural productivity. Both human activities and natural processes generate the chemical compounds that serve as
“precursors” to the formation of ozone. Currently human activities generate about as much ozone as natural processes do (Fishman 2002), creating
a public health hazard.

To solve the ozone pollution problem, governments have established ozone standards, revising them as new knowledge comes to light. The U.S.
Environmental Protection Agency (EPA) asserts that accumulated knowledge from 3,000 new studies of the last 15 years proves that breathing ozone
in smaller amounts than previously thought causes significant harm to human lungs. (U.S. Environmental Protection Agency, 1997). National
standards of many nations may need adjustment.

Governments enact legislation to enforce adherence to pollution standards. They also support ozone-monitoring activities in key parts of the
world, and fund research to enhance our ability to forecast ozone levels. But in spite of standards, regulations, monitoring and research, the
ozone problem persists. We are not only creating more ozone in some populated areas, but our ozone pollution crosses political boundaries and
often reaches across the Earth’s widest oceans. To regulate air quality, we need international cooperation and a strong scientific basis on
which to make legally binding agreements. Solving the problem also requires a shift in the ways we get and use energy. Everyone has a role to
play in regaining healthy levels of ozone in the air we breathe.

Red alder, Alnus rubra, shows a typical symptom of overexposure to ozone: discoloration of small groups of cells between the
veins, appearing as uniformly sized red to brown or purple spots (stippling). Mature leaves show more stippling than young ones, usually only on
the upper side of the leaf. Several common plant species respond to ambient levels of ozone pollution with visible symptoms that are easy to
diagnose (biomonitor). (Photograph courtesy of Pat Temple,
U.S. Forest Service).

Paul Crutzen has been one of the world’s leading researchers in mapping the chemical mechanisms that determine the
ozone content in the troposphere. Along with Mario Molina and Sherwood Rowland, Crutzen received the Nobel Prize for his stratospheric ozone
research in 1995. (Photograph courtesy of Paul Crutzen)